Plentiful food has made it easier than ever before to survive and reproduce in many parts of the world, so it's tempting to think that our species has stopped evolving. But a controversial new study says that isn't so. Far from slowing down, human evolution has sped up in the past 40,000 years and has become 100 times faster in the past 5000 years alone, according to the analysis. This means that even though some people have been globe-trotters who interbreed, most humans on different continents are becoming more different, rather than blending together into one genetically homogenous race.
In the current study, a team of researchers led by paleoanthropologist Henry Harpending of the University of Utah in Salt Lake City analyzed DNA from 270 individuals in the International HapMap Project, an effort to identify variations in human genes that cause disease. The team searched for single-nucleotide polymorphisms (SNPs)--mutations in an allele that spread throughout a population--and scanned sequence data from Europeans, Africans, and Asians. The researchers searched for SNPs that were flanked by tens of thousands of bases of identical DNA in many individuals in a population, because this suggests that the mutation is advantageous and under recent selection pressure to be preserved in a lineage.
Evolution has accelerated in 1800 human genes, which encompass about 7% of the human genome, Harpending's team reports online this week in the Proceedings of the National Academy of Sciences. Most of the mutations resulted from dramatic population booms, they suggest. As populations expand, the number of mutations increases, boosting the chances for a beneficial genetic variant that can improve survival and sweep through a population (in the same way that a large population of insects develops a gene for resistance to a pesticide faster than a small population).
Although the researchers don't know the identity of most of the genes, they say quite a few appear to be responses to changes in diet and a new wave of virulent diseases that swept through human populations as they began farming. Some examples include mutations that allow adults to digest starch, fatty acids, and lactose in milk, including mutations that arose in Europeans. Others improve the resistance to diseases, such as malaria, AIDS, and yellow fever in Africans. Several genes related to the production of human sperm also have been under selection in the past 10,000 years. Overall, "the pace of change has accelerated a lot in the last 40,000 years, especially since the end of the Ice Age," says Harpending.
The findings are persuasive to anthropologist Clark Larsen of Ohio State University in Columbus. But not everyone is on board. "I don't deny recent rapid selection," says geneticist Kenneth Kidd of Yale University. "But I am not yet convinced that so much rapid selection at so many places in the genome has occurred. ... I think we need much more data."